It's a bit complicated, that's why I left it out. But I can explain it here. In short, it was a result of many coincidences.
First let me explain the political arrangement in Holy Roman Empire. The electors would elect King of the Romans who would become emperor of the Holy Roman Empire when crowned by the Pope. But the latter did not always happen because the King of the Romans would either not be able to travel to Rome for the ceremony or was in openly hostile relationship with the Pope.
The last Holy Roman Emperor who was also actually crowned by the Pope before Frederick III was Sigismund of Luxembourg. He was a powerful ruler who became King of Hungary and Croatia in 1387 and King of Bohemia in 1419. In 1410 he was elected King of the Romans, and in 1433 he was crowned emperor in Rome. But he died soon in 1437.
Sigismund's heir was the Habsburg Duke of Austria Albert the Magnanimous who was married to his daughter Elizabeth of Luxembourg, Sigismund's only child. Albert became King of Hungary, Croatia and Bohemia. Even though his position was not as strong as Sigismund's and had to be fighting with nobility a lot, Albert was technically a king of all those lands, on top of also being Duke of Austria. That's why the electors elected him as the next King of the Romans in 1438. He had the strongest position in Holy Roman Empire and was also spirited and warlike, taking the fight to the Ottomans as the ruler of the borderlands of Christendom.
At the time his fellow Habsburg relative Frederick ruled Styria, Carinthia and Carniola (these lands were collectively called Inner Austria) which were separate from Duchy of Austria.
But Albert died soon in 1439. His only heir was his son Ladislaus who was still in his mother's womb at the time. He would become known as Ladislaus the Posthumous as he was born only after his father died and inherited his lands after that. But he was still just a child so the arrangement was made that Frederick would rule as a regent and be his guardian. This role, on top of already being duke of the lands I mentioned earlier, made Frederick the strongest Habsburg at the time when the Habsburg possessions were still split. But he had no real power in Hungary or Bohemia. He also had a conflict with his brother, also named Albert.
In 1440 Frederick was then elected as the new King of the Romans. Why? Well as you can see he had the most clear continuity with Sigismund of Luxembourg and Albert the Magnanimous through Ladislaus the Posthumous as he was his guardian.
You also have to keep in mind that the electors wouldn't necessarily select the most powerful ruler in HRE as Emperor anyway. They often went for the safe option. This is how for example Rudolf of Habsburg became King of the Romans in 1273 as the first Habsburg King of the Romans (he would never be crowned Emperor, though) over the powerful but aggressive and unpredictable King of Bohemia Ottokar II.
However unlike the previous Habsburg Kings of the Romans, Frederick also ended up being crowned as Emperor in 1453 as the Pope invited him to Rome to crown him. Why would the Pope do that? I would assume the 1448 Concordat of Vienna had a lot to do with that. It was a treaty signed by Frederick III that gave Pope a lot of power in the Holy Roman Empire, thereby basically ending the conflict between Empire and the Papacy which was the reason why Popes didn't crown many of the Kings of the Romans as Emperors in the past.
Frederick was favorable to the Pope and the Pope didn't have any reason not to crown him. Making such a grand ceremony was beneficial for both.
So you can see how through a series of lucky coincidences, a provincial duke ended up being crowned Emperor in Rome, the last one to have that honor (Maximilian was never crowned by the Pope and Charles V was crowned in Bologna).
What happened to Ladislaus the Posthumous though? Well, Frederick outlasted him as well. Ladislaus died already in 1457 and Frederick officially became the Duke of Austria. He outlived all the other Habsburgs and essentially united the Habsburg possessions.
You never mention inheritance Frederick secured with Count Cille (Celje) as they were involved also in story of Ladislav Hunyadi, brother of Mathias Corvin. He got a lot of lands in karinthia, styria and croatia.
Nice post - I am wondering how Frederick III managed to become Holy Roman Emperor considering the weak position you describe here?
It's a bit complicated, that's why I left it out. But I can explain it here. In short, it was a result of many coincidences.
First let me explain the political arrangement in Holy Roman Empire. The electors would elect King of the Romans who would become emperor of the Holy Roman Empire when crowned by the Pope. But the latter did not always happen because the King of the Romans would either not be able to travel to Rome for the ceremony or was in openly hostile relationship with the Pope.
The last Holy Roman Emperor who was also actually crowned by the Pope before Frederick III was Sigismund of Luxembourg. He was a powerful ruler who became King of Hungary and Croatia in 1387 and King of Bohemia in 1419. In 1410 he was elected King of the Romans, and in 1433 he was crowned emperor in Rome. But he died soon in 1437.
Sigismund's heir was the Habsburg Duke of Austria Albert the Magnanimous who was married to his daughter Elizabeth of Luxembourg, Sigismund's only child. Albert became King of Hungary, Croatia and Bohemia. Even though his position was not as strong as Sigismund's and had to be fighting with nobility a lot, Albert was technically a king of all those lands, on top of also being Duke of Austria. That's why the electors elected him as the next King of the Romans in 1438. He had the strongest position in Holy Roman Empire and was also spirited and warlike, taking the fight to the Ottomans as the ruler of the borderlands of Christendom.
At the time his fellow Habsburg relative Frederick ruled Styria, Carinthia and Carniola (these lands were collectively called Inner Austria) which were separate from Duchy of Austria.
But Albert died soon in 1439. His only heir was his son Ladislaus who was still in his mother's womb at the time. He would become known as Ladislaus the Posthumous as he was born only after his father died and inherited his lands after that. But he was still just a child so the arrangement was made that Frederick would rule as a regent and be his guardian. This role, on top of already being duke of the lands I mentioned earlier, made Frederick the strongest Habsburg at the time when the Habsburg possessions were still split. But he had no real power in Hungary or Bohemia. He also had a conflict with his brother, also named Albert.
In 1440 Frederick was then elected as the new King of the Romans. Why? Well as you can see he had the most clear continuity with Sigismund of Luxembourg and Albert the Magnanimous through Ladislaus the Posthumous as he was his guardian.
You also have to keep in mind that the electors wouldn't necessarily select the most powerful ruler in HRE as Emperor anyway. They often went for the safe option. This is how for example Rudolf of Habsburg became King of the Romans in 1273 as the first Habsburg King of the Romans (he would never be crowned Emperor, though) over the powerful but aggressive and unpredictable King of Bohemia Ottokar II.
However unlike the previous Habsburg Kings of the Romans, Frederick also ended up being crowned as Emperor in 1453 as the Pope invited him to Rome to crown him. Why would the Pope do that? I would assume the 1448 Concordat of Vienna had a lot to do with that. It was a treaty signed by Frederick III that gave Pope a lot of power in the Holy Roman Empire, thereby basically ending the conflict between Empire and the Papacy which was the reason why Popes didn't crown many of the Kings of the Romans as Emperors in the past.
Frederick was favorable to the Pope and the Pope didn't have any reason not to crown him. Making such a grand ceremony was beneficial for both.
So you can see how through a series of lucky coincidences, a provincial duke ended up being crowned Emperor in Rome, the last one to have that honor (Maximilian was never crowned by the Pope and Charles V was crowned in Bologna).
What happened to Ladislaus the Posthumous though? Well, Frederick outlasted him as well. Ladislaus died already in 1457 and Frederick officially became the Duke of Austria. He outlived all the other Habsburgs and essentially united the Habsburg possessions.
Thanks for taking the time to answer, very interesting. Will be following your story of the Habsburgs :)
Fantastic! Keep up the good work!
You never mention inheritance Frederick secured with Count Cille (Celje) as they were involved also in story of Ladislav Hunyadi, brother of Mathias Corvin. He got a lot of lands in karinthia, styria and croatia.